420 LECTURE XVIII. 



primordial cells, at first round, and provided with one nucleolus, 

 become afterwai'ds elliptical, and generally two nucleoli can be dis- 

 cerned in them ; afterwards two cells exist, of smaller size than the 

 parent cell. He concludes, that this fissiparous generation of cells, 

 which accords with that observed by Siebold and Bagge in the 

 Ascaris, is the general mode of their multiplication : — " Hgec omnia, 

 etsi nunquam cellulas in aliis inclusas oiFendi, ne ad sententiam 

 adducunt, posteriores a prioribus gigni, ita semper bina3 in unaque 

 cellula matre oriantur. 



The vitelline mass becomes elongated and vermiform, and, by 

 further subdivision and coalescence of the peripheric stratum of the 

 derivative germ-cells (" cambium " of Herold), a transparent integu- 

 ment is formed, like that in the Entozoon, first along the ventral 

 aspect, then ascending up the sides to the dorsal aspect, which is 

 likewise closed in by the reciprocally approximating folds which 

 cover first the cephalic and then the caudal segments. The portion 

 of the germ-mass remains long unchanged in the anal segment of the 

 larva of the bee. No part of the yolk can be properly said to enter 

 or be taken into the body of an insect. It never was out of the 

 body : it is a " germ-yolk ; " and forms the basis of the future body : 

 there is no appended or superadded vitellus, as in the shark or bird. 

 The division of the integument into the thirteen segments commences 

 at the ventral aspect, which is convex, the vermiform body of the 

 embryo being, at first, bent backwards. 



In the capitate larvae the entozoal type is quickly left by the 

 cervical constriction, and the development of a distinct head, which 

 commences by the formation of the part afterwards retained as the 

 labrum. The mandibulae and antennae next appear behind the 

 labrum as convex lobes ; and the part of the head in the lower inter- 

 space of the mandibles forms the labium : the maxillae then bud forth 

 between the labium and the mandibles. The median fissure, thus 

 sui'rounded by the rudimental trophi, sinks deeper into the substance 

 of the head, and, meeting a slender anterior production of the in- 

 ternal vitelline sac or cavity, establishes the mouth and oesophagus. 

 "Whilst these stages are in pi-ogress, the peripheral series of included 

 vitelline cells have undergone a series of spontaneous fissions ; 

 whereby the remaining mass becomes included within a second 

 stratum or cambium, which, by coalescence and further metamor- 

 phoses of the cells, is transformed into the tunics of the alimentary 

 canal, the interspace between which and the outer integument forms 

 the abdominal cavity. A certain proportion of the vitellus, not in- 

 cluded in the ellipsoid alimentary canal, has undergone transforma- 

 tions, by which the foundations of the muscular system, the ventral 



